1. Field of the Invention
The present invention relates to the field of Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), and, more particularly, to carrier phase measurements in such systems.
2. Description of the Related Art
Global Navigation Satellite Systems (GNSS) use a constellation of dispersed satellites with atomic clocks orbiting the Earth that transmit predictable signals at exact times. The modulation used by these signals and the data messages included enable the receivers to determine highly accurate navigational locations anywhere on the Earth. The receiver calculates its position by carefully measuring the time of arrival of the signals sent by several of the satellites. Each satellite continually transmits messages containing the time the message was sent, precise orbital information, and the general system health and approximate orbits of all the satellites. By calculating the difference between the broadcast “transmit time” and the received “time of arrival,” a time of propagation can be determined and transformed into a range using the speed of propagation “c”.
GNSS is considered a dual-use technology, namely, a technology that has significant civilian and military applications. Accordingly, for an example GNSS like the Global Positioning System (GPS), the satellites broadcast on precisely defined carrier frequencies with well-defined modulation. The GPS data and timing signals intended for everyone's use have a publicly-defined format contained in Interface Specification IS-GPS-200, available at http://www.navcen.uscg.gov/gps/geninfo/IS-GPS-200D.pdf, and are unencrypted, while those timing signals intended for military use are not publicly defined and are encrypted and the military specific information content is also encrypted. The satellite employs a pseudorandom code, which is used to modulate the carrier frequency in order to transmit the precise time marks. The carrier frequencies are over 1 GHz, while the code rates are considerably lower. GPS chip rates are roughly 10 MHz for the military code and 1 MHz for the civilian code. Additionally, data messages containing satellite orbit, system health, and other necessary information, are transmitted at even a lower rate of 50 bits per second.
Most conventional civilian navigational systems receive a GNSS signal through a single element fixed reception pattern antenna (FRPA) coupled to the receiver. Many military systems, however, use a multiple element controlled reception pattern antenna (CRPA) system to receive a GNSS signal. CRPA systems are much more resistant to the effects of intentional jamming of the GNSS frequencies than are FRPA systems and the signals from each of the elements can be coherently added to increase the carrier-to-noise-density ratio (C/NO) over that of a conventional FRPA type antenna for each received signal.
With GNSS, the receiver measures the transit time, using the precise time marks provided by the pseudorandom code, from a satellite and computes the distance to that satellite by multiplying the transit time by the speed of light. These distance computations are called “pseudoranges” since there is almost always a time difference between the atomic satellite clocks used to create the precise time marks and the receiver clocks used to decode the precise time marks. This clock error is common to all measurements since the atomic satellite clocks are all synchronized, and results in a common range error. This common range error is what forms a “pseudorange” from an absolute range. Other effects that give rise to range errors include atmospheric and receiver antenna hardware.
Geometric multilateration is used to pinpoint the receiver's location by combining these pseudoranges with the corresponding locations of the satellites, using the data from at least four different satellites. Four pseudoranges also allow determination of the clock bias associated with the common range error described above, which adds a fourth dimension of uncertainty, when trying to solve for the other three dimensions of a physical location. Nonetheless, other effects that contribute to range measurement errors still remain. Identifying and attempting to account for the multiple sources of errors is an important step to improving the accuracy of locations determined through GNSS.
Atmospheric (i.e., ionospheric and tropospheric) conditions are usually the next most significant source of error. The Earth's atmosphere slows down the speed of the satellite transmissions. These errors can be compensated for in a number of ways. For instance, using satellites that are more directly overhead helps because their transmissions travel through less atmosphere than when using satellites closer to the horizon. In addition, having the satellites transmit on multiple frequencies helps mitigate the ionospheric induced errors since it is frequency-dependent, so can be mitigated by combining the measurements from the two frequencies into a single ionospheric free measurement. Finally, relative posititioning systems, such as Differential GPS, use strategically placed monitor stations at exact locations to determine at any given time what the overall transmission delay (including effects like atmospheric) is for each satellite. These monitor stations then broadcast these delays to all nearby receivers, which then can make the corrections to each of the corresponding satellites.
There are still other effects, most notably receiver antenna hardware, which cannot be compensated through any of the above techniques. To the extent that such effects are not common between different satellites (common errors disappear as part of the clock bias correction calculated when determining location), they can affect the accuracy of the resulting positional calculation. Multiple element receiver antennas add complexity to the mitigation of these non-common errors, because the different elements receive and process the satellite signals with different hardware. Each hardware path contributes a different delay to the overall measured time of reception. Accounting for these more complex differences helps systems using multiple element antennas achieve the same accuracy that single element antennas are capable of achieving.
Because the satellite signals are relatively weak, it is fairly straightforward to intentionally jam such signals, either by increasing the noise floor by transmitting a broadband noise jammer or by attempting to exceed the dynamic range of the receiver hardware with powerful narrowband signals. Additionally, since the satellite signal structure is so precisely defined and predictable, it can be spoofed by a transmission using the same frequencies and signal structure. This is unacceptable for military applications, so they rely on encrypted signals to thwart any spoofed transmissions, but are still susceptible to intentional interference on the same frequencies. Consequently, for military applications, there is a need to reduce the effect of jamming, so the CRPA system is sometimes used in place of the FRPA system.
Intentional interference is usually significantly stronger than actual satellite transmissions. CRPA systems can use techniques such as nulling (combining the signals received by the CRPA's elements in such a way as to make the jamming signal cancel itself out) or beam steering (combining the signals received by the CRPA's elements in such a way as to amplify the satellite signal) to overcome intentional jamming. Note that beam steering doesn't physically direct the antenna hardware, rather it uses phased array techniques to compensate for the phase of arrival difference caused by the different path length to each element from any satellite to make the signals from each antenna element phase coherent so they add together in amplitude. Also note that it is possible to perform nulling and beam steering at the same time.
GNSS carrier frequencies (for example, military GPS receivers use two carrier frequencies, L1=1.57542 GHz and L2=1.2276 GHz, and a third GPS frequency is being added, L5=1.17645 GHz) have very short wavelengths, for instance, GPS L1 has a wavelength of 19.0 cm while GPS L2 has a wavelength of 24.4 cm. Sophisticated equipment can resolve down to a fraction of these wavelengths, producing extremely precise and accurate range measurements. The problem is that the phase center of reception of such waves can be difficult to determine for multiple element antennas tracking satellites from different angles. Without compensation, this effect contributes error when performing GNSS carrier phase measurements.
GNSS carrier phase measurements should be compensated for receiver hardware and directionally dependent antenna errors to obtain desired accuracies for high precision GNSS positioning applications. High accuracy carrier phase correction techniques implemented in the measurement domain have existed for a number of years. See, for example, Gerald L. Mader, GPS Antenna Calibration at the National Geodetic Survey, available at http://www.ngs.noaa.gov/ANTCAL/images/summary.html, the entire content of which is herein incorporated by reference. These techniques are usually based upon the GPS antenna phase-correction methodology pioneered by National Geodetic Survey (NGS), and are primarily applicable to GPS receivers that employ FRPAs. See the NGS home page at http://www.ngs.noaa.gov/ANTCAL/ for further FRPA error correction technical background.
To obtain the highest accuracy from GNSS carrier phase measurements, non-common receiver hardware induced errors and antenna induced errors dependent upon line of sight (LOS) angle to the satellite (azimuth and elevation) must be removed by compensation of the carrier phase measurements. This problem is not as significant an issue in FRPA GNSS sensors because it is straightforward to solve by subtracting the directionally dependent antenna errors from the carrier phase measurements, as disclosed by Mader above. However, for the complex case of a GNSS receiver employing a CRPA and dynamic beam steering, the multiplicity of combinations of antenna element outputs makes compensation more difficult, as the simple subtraction used for FRPA compensation does not work with a CRPA. Compensation of carrier phase measurements for such errors is a problem not addressed in previous GNSS CRPA beam steering sensors.
Therefore, with the conversion from the FRPA based systems to the CRPA systems for GNSS applications, there is a need to better compensate for the effects of antenna element errors on carrier phase measurement errors.